1. Field of the Invention
The present invention relates to oceanographic measuring systems, and more particularly to surface current mapping systems.
2. Description of the Prior Art
Surface currents are defined as the means horizontal flow of water within the uppermost layer of the ocean. The thickness of this layer can be nominally taken as 20 to 50 centimeters. On the open oceans, the height of the waves present will almost always be many times greater than the thickness of this top layer; in addition, the phase velocities of these waves will also be much larger than the speed of the mean surface currents (e.g., 10 to 100 times greater). Because of these two facts, the moored current meter--which is used successfully at greater depths--is relatively useless for measuring surface currents.
Yet the currents in the uppermost layer of the ocean are of great importance, especially in near coastal regions. Anything that floats on the surface is transported by these currents, waves notwithstanding. Thus the trajectory and fate of an oil spill or a leak from an offshore rig will depend upon the patterns of the near surface currents in the area, as will the destination of hot water and pollutant effluents discharged near the shore. The rescue of a person in the water, especially in conditions of poor visibility, could be aided considerably by real time observations of surface currents over the area and relevant trajectory predictions.
Nearly all available techniques for measuring these near-surface currents are Lagrangian in nature, meaning that they measure the trajectory of a parcel of water near the surface, thus obtaining one or more current streamlines vs. time. The most common technique consists of visualy or photographically observing a dye marker's dispersal or the movement of timed release floats from an airplane. Such experiments are expensive and hence limited in area and time. Drogued free floating buoys, tracked either by by radar or optically from land, ship, and possibly in the future by satellite represent an alternative technique. Satellite tracking of several such buoys will ultimately provide valuable information on general oceanic circulation patterns and thence, surface wind and ocean/atmosphere energy exchange, but can hardly be useful for the finer gridscale requirements for coastal waters over the continental shelves.
Observation of sea echo with HF radar has been employed as a method for measuring surface current features. HF, as considered here, extends from the broadest band to VHF, including radar wavelengths between 10 and 200 m. Although the heights of ocean waves are generally small in terms of these radar wavelengths, the scattered echo is nonetheless surprisingly large and readily interpretable in terms of its Doppler features. Sea currents are evident in the records as an overall offset of the Doppler peaks from their expected (normalized) positions at .+-.1. Barrick et al. in Proc. IEEE, Vol. 62, pp. 673- 680 and Stewart and Joy in Deep Sea Research, Vol. 21, pp. 1039-1049 have shown that the radial component of surface current, that is, the apparent surface current magnitude observed along the direction of a fairly narrow azimuthal radar beam, can be measured to a reasonable precision. A long, 800 foot, antenna array was used to form the beam so that the azimuthal direction of arrival could be assumed to be the same as that of the transmitted beam. The complete surface current vector could not be determined by this method. Also, these prior efforts did not permit real-time determination of the surface current since the data was recorded in the field and analyzed later.